Wave energy converter

ABSTRACT

A wave energy harnessing converter ( 1 ) has a tube ( 3 ) which floats on the sea. A water inlet ( 2 ) delivers water to the tube, and an air inlet ( 2 ) delivers air to the tube ( 3 ). The tube has sufficient buoyancy and flexibility to float on the water and conform to the shape of waves when the tube extends substantially in the direction of travel of the waves, causing water in the tube to be conveyed from the inlet and to be pressurised and the air to be compressed in a series of moving air pockets. The tube is reinforced to minimise energy losses through distortion or elongation. A converter output section ( 10 ) for receiving water and compressed air from the tube ( 3 ) for providing energy.

INTRODUCTION

1. Field of the Invention

The invention relates to a wave energy converter.

2. Prior Art Discussion

The oceans contain vast amounts of concentrated energy in the form ofwaves but harnessing this energy economically is very difficult.

A typical approach to wave energy harvesting is to provide anoscillating water column which pumps air through a turbine.

WO2008/036141 (Catlin) describes an ocean power harvester having anetwork of inter-connected semi-submerged devices with air compressors.

The invention is directed towards providing an improved converter andmethod for harvesting wave energy, which is more efficient, and/or morerobust, and/or simpler.

SUMMARY

According to the invention, there is provided a wave energy convertercomprising:

-   -   at least one tube to float on the sea or other water body,    -   a water inlet for delivering water to the tube,    -   an air inlet for delivering air to the tube,    -   wherein the tube has sufficient buoyancy and flexibility to        float on the water and conform to the shape of waves when the        tube extends substantially in the direction of travel of the        waves, causing water in the tube to be conveyed from the inlet        and to be pressurised and the air to be compressed;    -   a system output section for receiving water and compressed air        from the tube for providing energy.

In one embodiment, the tube has a curved cross-sectional shape.

In one embodiment, said tube has a diameter is in the range of 100 mm to2 m.

In one embodiment, the diameter is in the range of 500 mm to 1500 mm.

In one embodiment, the length of at least one tube is in the range of100 m and 1000 m.

In one embodiment, the length is in the range of 200 m to 600 m.

In one embodiment, the tube has a longitudinal stiffener.

In another embodiment, the stiffener extends along the neutral plane ofthe tube.

In one embodiment, there is a pair of stiffeners, one on each opposedside of the tube.

In one embodiment, the converter comprises a plurality of juxtaposed andinterconnected tubes forming a tube assembly.

In one embodiment, the tube assembly comprises a skirt along at leastone side of the assembly to reduce air ingress under the tubes.

In one embodiment, the converter further comprises a tensioningmechanism for varying overall length of the tube in the horizontalplane.

In one embodiment, the tensioning mechanism comprises tensioning ropesextending between the ends of the tube, and a control mechanism toadjust the length of the ropes.

In one embodiment, the converter further comprises water outtake meansfor removing water from a location to define a plurality of tube stages,in which pressure of stages increases with distance from the inlet end.

In one embodiment, the converter further comprises a manifold betweenthe stages for routing of air and water between different tubes.

In one embodiment, there are progressively fewer tubes as pressureincreases.

In one embodiment, the tubes of the successive stages are arranged inparallel.

In one embodiment, the higher pressure stages are biased towards beinglocated centrally.

In one embodiment, the water and the air inlets are combined in acombined inlet comprising a mouth to receive water and air and buoyancymeans to position the combined inlet to receive air and water.

In one embodiment, the mouth is arranged to receive water from crests ofwaves.

In one embodiment, the mouth comprises a tapered or curved guide forguiding water into the mouth inlet.

In another embodiment, the guide extends downwardly below the mouthinlet.

In one embodiment, the mouth comprises a plate located to cut the top ofa wave to take advantage of the momentum of the forward-rotating portionof the water in the top of the wave.

In one embodiment, the water inlet comprises means for being partlysubmerged.

In one embodiment, the water inlet is in the form of a substantiallyvertical riser, and comprises a pumping means to pump water upwardlythrough the riser.

In another embodiment, the pumping means comprises a feedback link fromthe outlet section arranged to deliver compressed air to the riser toprovide an air lift pump, said link providing at least part of the airinlet.

In one embodiment, the feedback link includes an air storage tank, andthe storage tank is adapted to release air into the riser.

In one embodiment, the water inlet comprises an oscillating watercolumn.

In one embodiment, the air inlet comprises a one-way valve at an upperend of the oscillating water column.

In a further embodiment, the air inlet comprises a bellows.

In one embodiment, the inlet comprises a buoy for supporting thebellows.

In one embodiment, the air inlet comprises a floating air trap having aninlet valve and an outlet for pulsed air driven by rising waves.

In one embodiment, the air inlet and the water inlet are arranged todeliver air and water into the tube at a volume ratio of substantially60:40-1-1+/−6%

In one embodiment, the output section comprises a flow restrictor tobuild pressure of air and water in each tube.

In one embodiment, the flow restrictor is an electricity generator suchas a turbine.

In one embodiment, the output section comprises an air/water separator.

In one embodiment, the output section comprises at least one turbine.There may be an air turbine and a separate water turbine.

In one embodiment, the output section is adapted to feed water to areservoir.

In one embodiment, the output section is adapted to feed compressed airto an external entity.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:—

FIG. 1 is a cross-sectional diagram showing a wave energy converter ofthe invention;

FIG. 2 is a pair of perspective views showing a combined water and airinlet for the system;

FIGS. 3( a) and 3(b) are views showing construction of the tube;

FIGS. 4( a) to 4(f) are diagrams illustrating the manner in which waveenergy is harnessed by the system;

FIG. 5 is a diagram showing a system having serial stages of lowpressure, mid-range pressure, and high pressure, and FIG. 6 is a diagramshowing a parallel arrangement;

FIG. 7 shows the principle of use of a “lilo” arrangement of juxtaposedtubes to form a tube assembly;

FIG. 8 is a diagram illustrating a system in which there are submergedair and water inlets;

FIG. 9 is a diagram showing a water inlet with an oscillating watercolumn arrangement;

FIG. 10 is a set of diagrams showing how a system may be tensioned foroptimum control; and

FIGS. 11 and 12 are diagrams showing alternative outlet sections.

DESCRIPTION OF THE EMBODIMENTS First Embodiment, FIGS. 1-3

Referring to FIG. 1 a wave energy converter 1 comprises a combined airand water inlet 2 at a leading end of a flexible compressor tube 3 ofapproximately 250 m in length, and terminating in a power output section10. In this embodiment, the output section 10 has an air turbine 11using compressed air exiting the tube 3 for electricity generation, anda water turbine 12 for electricity generation from water exiting thetube under pressure. The system 1 is anchored to the sea bed by ananchor 15, however in other embodiments it may be anchored to astructure such as a wind turbine column.

The inlet 2 provides a sequence of water and air plugs. The air plugsare akin to air locks, except that they move along the tube. Wave actionon the tube 3 causes pressurisation of the water and compression of theair. The output section 10 comprises a level sensor and a separatorwhich maintain water level to control flow through the turbines.

Referring to FIG. 2 the inlet 2 comprises a water guide 20 comprising apair of curved vertical plates arranged to focus water near the crestsof waves into an inlet formed by a bottom plate 21 and side and topwalls 22 and 23. The bottom plate 21 skims water from the crests ofwaves. A wave moves along horizontally but the water moves up and downand within a wave body the water is circulating. Hence at the crest of awave the water is moving substantially horizontally in the direction ofthe wave, at the top of its circular motion. The inlet 2 is arranged toreceive this water and, in-between crests, air. Thus the water entersthe inlet 2 with a certain kinetic energy to propel it into the inlet 2and the tube 3.

The inlet 2 combines three actions: tapered channel, wave cutting, andpitching to achieve sufficient momentum to feed the tube 3. First theconcentrator plates 20 concentrate a wide wave front into a narrow widthwith an opening width of approximately 3 times the tube diameter. Thewave crests become higher and the wave troughs become lower. Secondly,the level of the horizontal cutting plate 21 can be say 1 to 3 m abovethe mean water level, due to buoyancy, not shown. This might be higherthan the tops of the surrounding sea waves, but because the curvedguides 20 have amplified the waves and we only want to take the top 20%or so, the plate 21 can be set high up. Thus the inlet 2 is slopeddownwardly from its front, giving extra acceleration at tube entry.Also, the top portion of the wave contains the maximum forward motion,and thus it ‘shoots’ into the rigid inlet 2 (at 5 to 10 m/sec) and oninto the flexible tube 3. The reason for a length of rigid tube beforeentry to the flexible working tube is that if it was flexible it couldcollapse after a water slug and not open up again until the next slug.Therefore insufficient air would be drawn in. An alternative to a rigidtube inlet would be a flexible tube with a coiled spring embedded in therubber to hold it open so that it will not collapse between water slugs.

The inlet 2 and other inlets of the invention are arranged to provide anintake air:water ratio of approximately 60:40 by volume (+/−6%). It hasbeen found that this is particularly effective due to progressivecompression of the air pockets as they are conveyed along the tubetowards the outlet.

Referring to FIGS. 3( a) and 3(b) the tube 3 has woven fibres 31 ofmaterial such as polyester or steel to provide excellent robustness andflexibility. Importantly, there is a stiffening rib 30 along the tube atopposite sides, at the tube's neutral bending plane. In some embodimentsthe system comprises multiple juxtaposed interconnected tubes arrangedas shown in FIG. 3( b). The arrows in these diagrams show clockwise andanti clockwise windings going down into the page, to illustrate thatthere are two separate sets of windings.

To maintain the diameter of the tube it is wrapped with embedded spiralreinforcements, as shown in FIG. 3( a), with half spiralling clockwiseand the rest counter clockwise. This arrangement is known in principlefor hoses such as industrial hoses. The spiralling reinforcements may bemade of any suitable filament type such as polyester.

If one employs longitudinal reinforcements evenly around the diameter ofthe tube under certain conditions the longitudinal reinforcements maytend to straighten, possibly leading to kinking. In mechanical andstructural engineering, the concept of a “neutral bending plane” is wellknown. The material neither stretches nor compresses at the neutralbending plan'. As shown in FIG. 3( a) by placing two longitudinalreinforcements on opposite sides of the diameter of the tube, they willtogether define a neutral bending plane. The longitudinal reinforcementsremain with a constant length while the tube is bent downwards. Thematerial, or matrix, stretches more as the distance from this neutralplane increases. The longitudinal reinforcements may be made of twistedsteel cable, Vectran™ or any suitable high modulus material.

It will thus be appreciated that the construction of the tube is suchthat diameter increasing under pressure and longitudinal stretching isavoided, while permitting vertical compliance with the sea waves.

Energy Conversion Mechanism

As shown in FIGS. 4( a) to 4(f) air is compressed in the flexible tubes,which float while following the surface of the waves. The tubes arefilled with sequential volumes of air and water in a pattern whichmatches the waves. Air tends to rise to the parts of the tubes floatingat the crests of the waves while water tends to gather at the hollowsbetween the waves. Both air and water move within the tubes at a speedwhich matches the movement of the waves. If no restriction is placed onthe outflow from the tubes, they would simply act as low pressure pumpsconveying the water and air in the direction of the waves. If, however,restrictions such as electricity-generating turbines are placed at theoutlets of the tubes the water segments move backwards somewhat andopposite to the general flow. Small pressure heads are thus created ateach wave, each one like a small air lock moving towards the outlet. Thecumulative addition of all of these pressure heads generates asubstantial pressure at the outlet if the length of the tube issufficiently long. The air segments compress in volume as they movetowards the outlet, where the pressure is highest.

Air compression as described above is (desirably) isothermal, due to thecooling effect of the water and slow compression rate. Air volume ishalved when compressed to one atmosphere (1 Bar, gauge). Thus, insidethe tube 3 the volume of the air portion reduces as it moves downstream.When 1 Bar is reached the volume is halved, thus taking up anincreasingly large proportion of the tube volume with water which is notadding to the head, and therefore not being used to any advantage. FIGS.4( a) to 4(c) illustrate the progress of one water/air segment along atube. Initially, the air volume is large and only part of the water iscontributing to the working part of the compressor. As shown in FIG. 4(a) the head h1 is not perfect but satisfies a necessary condition toallow for progress down the tube. Moving onwards to the next stage (FIG.4( b)) there are optimal conditions where all the water is contributingto the head h50 and there is no water which is not contributing to thehead and thus the work of compression. But next we move on to h100 andsee the head has reduced again, there is backwards spill of water (butnot air) and energy is being lost in turbulence. This is a sub-optimalsituation in so far as it dissipates energy, but is necessary to achievehigh pressures. In irregular sea waves there will be a mixture of thesethree conditions happening along the tube at any given moment, with thecondition at FIG. 4( a) dominating at the beginning and at FIG. 4( c)dominating towards the outlet. To continue incrementally increasingpressure the system can remove much of the water so that the air/waterreturns to where it was at h1, but now at the higher pressure. Since theair volume has reduced by compression and water is removed, we needfewer tubes to continue the compression in a second stage, oralternatively a slower overall velocity which extracts more energy fromslower moving waves. This excess pressurised water can be collected fromall the tubes and usefully used. It is under useful pressure, equivalentto a head of about 10 m and therefore it can be used to compressatmospheric air to say 9 m head, and this can then be fed back upstreamand into the tubes where air at this pressure may enter. Alternatively,the water could be used in a turbine to generate power. The number ofworking tubes is preferably reduced when this water is removed, as wehave a reduced air volume coupled with water removal. While the firstbar of pressure above atmospheric leads to a reduction in air volume tohalf, a further pressure rise to 3 bar is required for the air volume toagain be halved. Thus, as pressures rises the need to remove excesswater reduces rapidly. Water take out therefore is predominately closerto the intake/low pressure end of the system.

FIGS. 4( d) to 4(f) illustrate the same length of tube 3 at timeintervals of a few seconds apart. At the left of FIG. 4( d) (above theformula) is the moving slug of water ‘W’. In FIG. 4( e) the same slug Whas moved to the right and in FIG. 4( f) it has moved further to theright. As the oncoming wave lifts the tube 3 the water flows to theright in front of the oncoming wave as if it were surfing the front ofthe wave. The air is trapped between successive slugs of water so thatit can not move backwards in the direction of the oncoming waves. Thistrapped air in a tube is normally referred to an air lock. In this casethe air locks are also being moved forward within the tube. Multiple airlocks have the potential to cause a large pressure difference betweenthe inlet and outlet of a tube. As the waves move the air locks towardsthe outlet the pressure builds up and up and is equivalent to the sum ofthe small pressure heads either side of each moving air lock. So, thepressure at the outlet is equal to the sum of all the air lockdifferentials within the tube.

It is preferred that the water has a speed at the inlet which matchesthat of the waves so that the water slugs ‘surf’ along in the tube in amanner analogous to a human surfer on a wave. As the water has usefulkinetic energy as it exits the tube a diffuser in the output section canbe used to convert this kinetic energy into extra pressure as it entersthe collection tank.

As the waves move the tube 3 in a manner to follow the general shapes ofthe waves the water inside the tube is pressurised as the air betweenthe slugs is compressed. As sea waves have a spectrum of speeds we tryto match the speed of maximum energy. For example on the inlet 2 theremay be a chamber in the buoyancy for adjustment of the level in thewater.

Where the inlet comprises an air lift arrangement as described below,air flow rate, depth of the riser tube, and bubble size for examplecould be controlled.

The converter ‘tunes’ to the wave spectrum by choosing the input speed.The tube has a relatively wide capture bandwidth. Inside the tube thevelocity remains fairly constant, only reducing slightly as the airportion compresses. Energy is pressure×volume, and as the volume flowrate remains approximately constant the pressure rises continually as ittravels along the tube.

The waves cause the tubes to move in a wave motion, transferring energyto the air and water in the tube. This energy takes the form ofcompression of the air and a rise in water pressure.

Power is energy/sec=pressure×flowrate. The tube 3 has two flows out,water and air. For the water with a cross sectional area of say 1 m² andvelocity of 5 m/sec, and coming out 50% of the time, there is a flow of1 m²×5 m/sec×½=2.5 m³/sec. If exit pressure is 1 bar (or 10 m H₂O)=100000 N/m2. Then the water power exiting the tube is 2.5 m3/sec×100 000N/m2=250 KNm/sec=250 KW. (1 Nm/sec=1 watt). The air portion is also 50%of the volume at exit and the same pressure×flowrate. Air could normallybe expected to exceed 250 kW for the same flow rate and pressure.

Arrangement of Multiple Tubes

Referring to FIGS. 5 and 6 the manner in which a system of the inventionwith multiple juxtaposed tubes may be arranged in low, medium, and highpressure sections is illustrated. In FIG. 5 a wave energy converter 50has first, second, and third compression stages 51, 52, and 53. As thepressure increases along the length of the tubes some back spill ofwater to the upstream air segment becomes inevitable (FIG. 4( c)),wasting some energy in turbulence, but also beneficially helping toreduce the air volume and increase the air pressure in that upstreamsegment. This dissipation of energy by back-spilling eventually leads toa substantial fall off in energy gathered per unit length of tube. Thus,for improved efficiency, a second stage compression is preferablewhereby some water is removed for enhanced step-by-step progress towardshigher pressures. Two and three stage compression is sometimes used instandard air compressors; with inter-cooling between stages, in order tobetter approximate isothermal compression. In this case however, thereis isothermal compression and the need for more than one stage ofcompression is to re-establish the optimum air:water ratio and thusoptimise gathered energy per unit length and to reduce back-spill energylosses. As shown in FIG. 5 the stages may be sequential, whereas asshown in FIG. 6 they may be parallel, with the high pressure stagepreferably located in the centre. There may be interleaving of high,medium and low pressure tubes, overall having a higher concentration ofhigh pressure towards the centre.

Multi stage compression, aimed at achieving higher pressures, benefitsfrom the lack of air underneath the tube assembly. This means that theatmospheric pressure presses the tubes against the water surface(“suction effect”). The outer tubes of the assembly may be assignedfirst stage compression duties of, say, up to one Bar, while some innertubes, towards the centre, may be carrying out second or third stagecompression duties of several Bar. They are therefore stiffer, but wherethe suction effect is at its most dependable and best able to counteractthis extra stiffness.

As described above with reference to FIG. 6 second and third stagecompression stages may be in parallel and within the same tube assemblyas the first low pressure stage and not in series, or downstream asillustrated in FIG. 5. This will mean returning pressurised air and somewater back upstream to near the air intakes. This may be carried backupstream in straight pipes at a much higher velocity than the velocityin the working tubes, and would therefore be of much smaller diameter;this diameter being only sufficient to avoid excessive friction losses.In one arrangement, this pressurised air and water travels back upstreamthrough tensioned span limiters, consisting of hollow pipes undertension. The system can be arranged so that one span limiter would carrypressurised water only, while another carries air only. As these spanlimiters are very long the buoyancy of the air pipes may be useful as astructural support for the water pipes.

An advantage in combining in a parallel arrangement first stagecompression and higher pressure stages in the one tube assembly is tomake best use of the “suction effect”. The higher pressure tubes shouldbe located to dominate towards the centre, to reduce edge lifting andair ingress at the outer edges, as shown in FIG. 6.

Referring to FIG. 7, tubes may be arranged juxtaposed in a lilo-likeassembly 100 of tubes 101. There are preferably side skirts 103 tominimise unwanted air ingress from the sides. In one embodiment, thearrangement is 250 m long by 25 m wide, with each tube having a diameterof about 1 m. FIG. 7 also shows one of many diverter films 102 fordiverting water off the assembly 100. This avoids wasting energy byflowing water on top of the assembly 100.

Distinct individual tubes as described will work, but with huge areas tocover, and the need to withstand storm conditions, it may beadvantageous in some conditions to join the tubes side-by-side. Thisway, the encircling reinforcements also secure the longitudinalreinforcements and join the tubes into the “lilo” arrangement, asillustrated in FIG. 3( b). Also, as described in more detail below, theenergy extracted per meter per tube for this arrangement is higher thanfor individual tubes.

The reduction in energy extraction resulting from the tendency of thetubes to straighten and cut through the waves is substantially lessenedwith a wide lilo-like arrangement. Because the crests can not break upthrough the impervious lilo-like layer above and also, since air can noteasily find its way underneath there is a greater compliance of the widelilo arrangement to the wave shape than the single tube. Air attemptingto get underneath the lilo, where its edges meet the wave hollows, arefaced with a moving labyrinth seal. Also, suction is created in the wavehollows, if the tube assembly attempts to rise away from the wavehollows. This is referred to as the “suction effect”. Thus a wide tubeassembly is forced to comply with the wave pattern much more effectivelythan a single tube.

To enhance the “suction effect” the sealing skirts 103, on each side ofthe assembly are incorporated to block unwanted air ingress, as shown inFIG. 7. One or more skirts may incorporate non return flap valves on thedownstream end to evacuate any unwanted air ingress.

Alternatively or in addition a higher water:air ratio in the outermosttubes would also help to keep these down on the water in the wavehollows, thus helping to prevent air ingress, and preventing it gettingto the stiffer high pressure tubes towards the centre.

Embodiment with an Air-Lift Inlet (FIG. 8)

Referring to FIG. 8 a system 200 has a tube 201 and a submerged combinedair and water inlet 202 having a riser 203 having a length of 10 m. Thedepth can be chosen to a level to set a desired pressure of the entiresystem. It is calculated that for each additional 10 m riser depth thetube air pressure rises by 1 bar. There is a compressed air storagereservoir 204 for controlled delivery of compressed air to the riser203. The reservoir 204 is fed by a feedback air link 205 from the outletsection 210. The feedback air link may in some embodiments have anin-line air turbine for extraction of some of the air energy if thepressure available is greater than necessary to supply the riser 203.

Injection of compressed air into the bottom of the riser 203 provides an“air-lift” pump analogous to those sometimes used in the miningindustry. Advantageously, the system has compressed air available in theoutlet section 210. Fine air bubbles are introduced into the bottom ofthe riser. As the bubbles rise the combined density of air and water inthe vertical column is substantially lower than the outside waterdensity. Being lighter than the water it rises. The bubbles rise andcombine to form air slugs in the tube 201, leaving the water to form thewater slugs. The water already has velocity so this supplies thenecessary momentum as it enters the tube. This is a closed air circuit(with some top up to make up for dissolved air, and to control theoverall pressure in the system, as well as the air/water ratio). Thewater on the other hand is an open circuit.

There is considerable kinetic energy in the riser, helping to maintainflow during a short lull in the waves. A major advantage is that theinlet of the tube is not exposed to storm damage. An approaching wavewould only ‘see’ the tubes rising up in a curve from below. The air liftpump would be mainly located below the most powerful wave action. Thecurved tube would present a small resistance to large oncoming waveswhich would tend to pass over.

The reservoir 204 can be filled with high pressure air during energeticwave conditions from tube outlets, or it could be topped up with airfrom a compressor powered by a wind turbine or other type of wave energyconverter.

There may in some embodiments be a means to vary the depth of the airinlet to avoid stalling the output from the tubes. There may be morethan one air in-feed in the riser, to select the level (or pressure) ofair input.

Embodiment with Oscillating Water Column (“OWC”) Inlet. (FIG. 9)

Referring to FIG. 9 a system 300 comprises an oscillating water column301 with an air valve 302 above the column, and a tube 303. It is knownto provide an oscillating water column having a Wells air turbine toextract the energy. In the OWC 301 there is water overtopping and airpumping via the non return valve 302. Both of these feed air and waterto the tube 303. In more detail, there is overtopping on the upswing ofthe oscillating water, followed by air suction on the downswing. On theupsurge the displaced air is blown into the tube followed by overtoppingwater which drives the entrained air into the tube 303 before it. Thenas the oscillating water falls back the air is again drawn in throughthe non return air inlet valve. As the water swings up again the cyclerepeats itself.

Referring to FIG. 10, a system 400 has a tensioning mechanism 401 whichwinds lengths of stiffening cable 403 to set the length of the tubeassembly in the horizontal plane.

Tubes tend to straighten when pressurised and overcoming this tendencyis advantageous. Otherwise, the tubes may not follow the wave shapesufficiently well, and may tend to ride on top and through the waves,thus gathering little energy. For most efficient harnessing of waveenergy the tubes must follow the wave pattern reasonably closely.

The reinforced tubes as described would, if closed at both ends andpressurised, flex up and down easily. If anchored at one end and filledwith sea water to give an overall density less than but close to thedensity of sea water, it would follow the waves up and down closely.However, if the outlet end of this water-filled tube was forcefullypulled, the up and down waveform would straighten out, cutting into thetops of the waves and hanging free of the wave hollows. The sinusoidalamplitude would lessen. The potential ‘heads’ obtainable within thisshape tube would be less than if the tube more accurately followed thewaveform. The efficient use of tube material would be lessened, as ifthe seas were calmer than in reality. The Watts per meter would suffer.

As the system is not handling a tube filled with stationary sea waterbecause it is continually compressing air along the length, the force,(pressure×area) or tension grows as the internal pressure grows. Thishas an effect similar to catching the end of the water-filled tube andpulling against the anchor so that the tube will tend to cut into thecrests of the waves and hang above the wave hollows.

The tendency of the tubes to straighten and cut through the waves islargely solved by the longitudinal reinforcements along the neutralbending plane, and enhanced by the “suction effect” but to achieve yethigher pressures, an additional and different type of structure isadvantageously employed.

The tensioning mechanism (FIG. 10) prevents the exit ends of the tubesmoving away from the entry ends. The span (overall length of the tubeassembly as viewed in plan) is adjustable, depending on weather and waveconditions. In a very calm sea the wave height will be so small that thespan will almost become equal to the overall length of the tube. In highwave conditions however, these tension members are shortened to permit ahigh amplitude-pattern matching the waves. A shorter span is needed forhigher waves, but the span may be set at longer than optimal length toavoid the situation where the water in the tube is moving so quickly inrelation to the tube diameter that friction and turbulence becomeexcessive leading to a loss of power. A control system, detectingupstream wave conditions, and a motorised arrangement to permitadjustment of the span solves this problem. Thus, compliance with theshape of the waves is enhanced for all wave conditions and the energyintake per unit length is optimised. These long span control tensionersmay be coated steel cables as, they may serve a dual purpose and behollow straight pipes for carrying pressurised air and/or water.

Together, longitudinal reinforcements, the “suction effect”, spanlimitation, and a pliable matrix material improve waveform complianceand high pressure air compression.

Alternative Output Sections

Referring to FIG. 11, in a system 700 the exhaust water is fed to a highreservoir to provide a head for pumped storage power generation.

Referring to FIG. 12, compressed air may be pumped to a low water depth,causing it to rise as bubbles and causing what is known as up-welling.Up-welling is proposed as a way of raising the nutrient deep waters tothe surface where photosynthesis can take place. This up-wellingtechnique is proposed as a way of causing great masses of algae growthto be collected at a great distance from the up-welling and used asbiomass or fertiliser.

Alternative Embodiments

In most of the embodiments described both fresh air and fresh sea waterenter and leave the tubes. However, there are some situations where itmay be worthwhile to return the water in a closed loop for reuse, insuch a way that the system is an open system for air but a closed systemfor water. This almost eliminates the problem of having to filter thewater for seaweed before it enters the pipes. Filter screen cleaning isalso virtually eliminated. It also avoids the potential problem ofenergy losses due to air dissolving in the water, and so, not beingavailable for use as compressed air. When the oxygen content of thecompressed air is important for some applications, for oxygen productionor some combustion systems, a closed water system would reduce oxygendissolving in water and being wasted.

Regarding tube diameter, smaller diameter tubes have lower anchoragerequirements but also lower throughput in all but calm seas. Thematerial cost per unit power is higher. The choice of diameter isprimarily a compromise between wave conditions and tube friction losses.It also depends on whether one wants a reliable power supply at arelatively low level most of the time or maximum energy over time. Anisland, with no connection to a mainland power line and with limited orno storage might require power for the maximum number of hours per annumas opposed to the maximum overall energy.

A major advantage of the invention is that it lends itself to continuousprocess manufacture. There is no welding, chopping, plating, screwing,pivots, seals or like fabrication. This is very amenable for largescale, low cost, continuous manufacture. Ideally, the tube assemblywould exit from the production facility directly on to water,eliminating the need for very wide conveyors. A production facilitycould also be based on a ship that could travel to the designatedlocation and produce in situ, the ship being also used as operationalheadquarters, for ancillary production and assembly.

The converter tube or tubes are very tough and are flexible enough toyield under storm conditions as it is reinforced extensively but notrigid. The fact that air and water are both combined in the tubes meansthat potentially destructive oscillations are damped out. There is alsoscope to change the storm-resistant properties if bad storms areforecast, such as:

-   -   fill tubes mainly with air and change span control, and/or    -   fill with water mainly and adjust span control, and/or    -   inject air below the tube assembly to break the “suction        effect”.    -   Lower the inlet feed to allow larger waves to pass over

It will also be appreciated that the arrangement of the system lendsitself to a low maintenance requirement. Absence of metal parts meansthere is little corrosion.

Also, as the tubes lie so close to the water surface that they shouldnot be visible when just a few kilometres off shore. The ancillaryequipment, feed and power, are smaller than the tube assembly and canalso be designed to be low profile.

For fish conservation the need for marine reservations is well accepted.An energy farm employing a system of the invention and a marinereservation could advantageously co-exist. Some of the oxygen in thetubes will dissolve in the water given the pressure, time and movementinvolved. When this water is exhausted from a turbine this enrichedoxygen water would be available to marine life.

It will be appreciated that the invention overcomes the main potentialdifficulties such as storm damage, expense, maintenance in a hostileenvironment, visual impact, and high strain anchorage, and widebandwidth.

It is also envisaged that the outlet end of the tubes may be anchored onland or an island or structure such as an oil rig.

The invention is not limited to the embodiments described but may bevaried in construction and detail.

1-40. (canceled)
 41. A wave energy converter comprising: at least onetube to float on the sea or other water body, a water inlet fordelivering water to the tube, an air inlet for delivering air to thetube, wherein the tube has sufficient buoyancy and flexibility to floaton the water and conform to the shape of waves when the tube extendssubstantially in the direction of travel of the waves, causing water inthe tube to be conveyed from the inlet and to be pressurised and the airto be compressed; a system output section for receiving water andcompressed air from the tube for providing energy.
 42. The wave energyconverter as claimed in claim 41, wherein said tube has a diameter is inthe range of 100 mm to 2 m.
 43. The wave energy converter as claimed inclaim 41, wherein the length of at least one tube is in the range of 100m and 1000 m.
 44. The wave energy converter as claimed in claim 41,wherein at least one tube has a longitudinal stiffener.
 45. The waveenergy converter as claimed in claim 41, wherein at least one tube has alongitudinal stiffener which extends along a neutral plane of the tube.46. The wave energy converter as claimed in claim 41, wherein at leastone tube has a longitudinal stiffener on each opposed side of the tube.47. The wave energy converter as claimed in claim 41, wherein there is aplurality of juxtaposed and interconnected tubes forming a tubeassembly.
 48. The wave energy converter as claimed in claim 41, whereinthere is a plurality of juxtaposed and interconnected tubes forming atube assembly; and wherein the tube assembly comprises a skirt alongsides of the assembly to reduce air ingress under the tubes.
 49. Thewave energy converter as claimed in claim 41, further comprising atensioning mechanism for varying overall length of the tube in thehorizontal plane.
 50. The wave energy converter as claimed in claim 41,further comprising a tensioning mechanism for varying overall length ofthe tube in the horizontal plane; and wherein the tensioning mechanismcomprises tensioning ropes extending between the ends of the tube, and acontrol mechanism to adjust the length of the ropes.
 51. The wave energyconverter as claimed in claim 41, further comprising water outtake meansfor removing water from a location to define a plurality of tube stages,in which pressure of stages increases with distance from the inlet end.52. The wave energy converter as claimed in claim 41, further comprisingwater outtake means for removing water from a location to define aplurality of tube stages, in which pressure of stages increases withdistance from the inlet; and further comprising a manifold between thestages for routing of air and water between different tubes.
 53. Thewave energy converter as claimed in claim 41, further comprising waterouttake means for removing water from a location to define a pluralityof tube stages, in which pressure of stages increases with distance fromthe inlet; and wherein there are progressively fewer tubes as pressureincreases.
 54. The wave energy converter as claimed in claim 41, furthercomprising water outtake means for removing water from a location todefine a plurality of tube stages, in which pressure of stages increaseswith distance from the inlet; and wherein the tubes of the successivestages are arranged in parallel.
 55. The wave energy converter asclaimed in claim 41, further comprising water outtake means for removingwater from a location to define a plurality of tube stages, in whichpressure of stages increases with distance from the inlet; and whereinthe tubes of the successive stages are arranged in parallel; and whereinthe higher pressure stages are biased towards being located centrally.56. The wave energy converter as claimed in claim 41, wherein the waterand the air inlets are combined in a combined inlet comprising: a rigidtube having a mouth to receive water and air, the mouth having a bottomplate located to cut the top of a wave to take advantage of the momentumof the forward-rotating portion of the water at the top of the wave, andbuoyancy means to position the rigid tube to receive air and water withthe inlet tube sloped downwardly from its front.
 57. The wave energyconverter as claimed in claim 41, wherein the water and the air inletsare combined in a combined inlet comprising: a rigid tube having a mouthto receive water and air, the mouth having a bottom plate located to cutthe top of a wave to take advantage of the momentum of theforward-rotating portion of the water at the top of the wave, buoyancymeans to position the rigid tube to receive air and water with the inlettube sloped downwardly from its front, and wherein the inlet comprises atapered or curved guide for guiding water into the mouth.
 58. The waveenergy converter as claimed in claim 41, wherein the water and the airinlets are combined in a combined inlet comprising: a rigid tube havinga mouth to receive water and air, the mouth having a bottom platelocated to cut the top of a wave to take advantage of the momentum ofthe forward-rotating portion of the water at the top of the wave,buoyancy means to position the rigid tube to receive air and water withthe inlet tube sloped downwardly from its front, wherein the inletcomprises a tapered or curved guide for guiding water into the mouth,and wherein the guide extends downwardly below the mouth.
 59. The waveenergy converter as claimed in claim 41, wherein the water inlet is inthe form of a substantially vertical riser, and comprises a pumpingmeans to pump water upwardly through the riser.
 60. The wave energyconverter as claimed in claim 41, wherein: the water inlet is in theform of a substantially vertical riser, and comprises a pumping means topump water upwardly through the riser; and wherein the pumping meanscomprises a feedback link from the outlet section arranged to delivercompressed air to the riser to provide an air lift pump, said linkproviding at least part of the air inlet.
 61. The wave energy converteras claimed in claim 41, wherein: the water inlet is in the form of asubstantially vertical riser, and comprises a pumping means to pumpwater upwardly through the riser; and wherein the pumping meanscomprises a feedback link from the outlet section arranged to delivercompressed air to the riser to provide an air lift pump, said linkproviding at least part of the air inlet; and the feedback link includesan air storage tank, and the storage tank is adapted to release air intothe riser.
 62. The wave energy converter as claimed in claim 41, whereinthe water inlet comprises an oscillating water column.
 63. The waveenergy converter as claimed in claim 41, wherein the water inletcomprises an oscillating water column; and wherein the air inletcomprises a one-way valve at an upper end of the oscillating watercolumn.
 64. The wave energy converter as claimed in claim 41, whereinthe air inlet comprises a bellows.
 65. The wave energy converter asclaimed in claim 41, wherein the air inlet comprises a bellows; andwherein the inlet comprises a buoy for supporting the bellows.
 66. Thewave energy converter as claimed in claim 41, wherein the air inletcomprises a floating air trap having an inlet valve and an outlet forpulsed air driven by rising waves.
 67. The wave energy converter asclaimed in claim 41, wherein the output section comprises a flowrestrictor to build pressure of air and water in each tube; and whereinthe flow restrictor is an electricity generator such as a turbine. 68.The wave energy converter as claimed in claim 41, wherein the outputsection comprises an air/water separator.
 69. The wave energy converteras claimed in claim 41, wherein the output section comprises an airturbine and a separate water turbine.